1. Field of the Invention
The present invention relates to semiconductor image sensors. More particularly, the inventions described and claimed herein relate to a method for inspecting and testing a semiconductor image sensor wafer and device that has had a protective layer applied to its front surface.
2. Description of the Related Art
Semiconductor fabrication processes are well established. In semiconductor fabrication, semiconductor wafers are modified and transformed in a series of multiple processing steps to create desirable features on the finished semiconductor chip or finished module. Image sensors are semiconductor chips fabricated to receive and capture light image information (e.g., intensity information) and directly convert the captured light information to form a digital image. Image sensors are classified as charge-coupled devices (CCDs) and complimentary metal oxide semiconductor (CMOS) devices. CCD image sensors capture light onto an array of light sensitive diodes, where each diode represents a pixel. The pixel converts the light photons into commensurate electric charge, which is accumulated by a charge-coupling operation to generate each pixel's signal. CMOS image sensors, like CCD image sensors, include an array of photosensitive diodes, with one diode allocated for each pixel. They differ however in that each CMOS-based pixel includes its own amplifier and may be read directly on an x-y coordinate system rather than by the charge-coupling process required by CCD-based image sensors.
The reader should note that the percentage of a pixel area on the active wafer surface that actually captures light is called the pixel “fill factor.” This light capturing or active area is typically about 40% of the allocated pixel surface area. The remainder of the allocated pixel area surface is used for various purposes known to the skilled artisan. In some image sensor pixel designs, the fill factor is too small to be effective for particular imaging applications. To accommodate this problem, the image sensor allocated pixel area surface may include small lenses (i.e., microlenses) formed directly above each pixel. The microlens focuses the incident light received at the pixel area towards the active area or light-capturing portion of the allocated pixel surface area. This effectively increases the fill factor, for example, up to three (3) times.
Because the active or light-capturing portion of each pixel's allocated surface area (with or without microlenses) is light responsive, it follows that pixel operation is extremely sensitive to contamination, e.g., dust particles that may accumulate thereon, and surface damage such as scratching which may occur in various post-fabrication handling processes. The contamination and damage may block or limit accurate capture of the light incident on affected pixel areas. That is, contamination particles can block or modify incident light received thereon and sensed by the photodiodes comprising the image sensor. This can cause the individual sensors to fail testing, and may show up in an image acquired by a damaged or contaminated image sensor that nevertheless passes testing. For that matter, wafer testing may generate contamination such aluminum (Al) particles that are scraped up by probe needles, picked up and deposited elsewhere on the active or front surface.
Conventionally, image sensor fabrication and post-fabrication processes are carried out in clean rooms, or under clean room conditions to avoid contamination by dust and other debris. To avoid damaging the front surfaces with or without microlenses, finished wafers are stacked (stored) and transported using crystal packs, which increase manufacturing cost. Since wafer test assembly areas are typically not controlled to the same standards maintained within cleanrooms for wafer fabrication, contamination of image sensor wafers is a key challenge to high yield manufacturing. For that matter, merely stacking the image sensor wafers between processes can damage very sensitive microlenses.
To overcome such sensor surface contamination problems, U.S. patent application Ser. No. 11/559,983, commonly-owned and filed concurrently herewith, teaches a method that includes applying a protective tape layer to the active or front surface of the fabricated image sensor wafer. The protective tape is preferably a “zero-washing” tape, or any known tape normally used during fabrication. For example, back-grinding tapes that are normally used for application to a backside of a semiconductor wafer during grinding processes will effectively protect the front surface when applied post-fabrication. Protective tapes are preferably light transparent to facilitate inspection, testing, storage, cutting and dicing, and/or transportation to other locations for assembly, etc., to protect against front surface damage, or accumulated contamination that might otherwise affect sensor operation. That is, debris generated from the wafer testing, dust particles and the like accumulates on the protective tape layer rather than being deposited on the active sensor surface. Preferably after assembly to module form, the tape is merely peeled off the active light-capturing surfaces.
While the Ser. No. 11/559,983 application is known to protect the front surface of image sensor wafer post-fabrication by the application of the protective layer, there is no method or process known which allows for inspection and quantification of the quality of the wafer after the protective layer is applied. It would be desirable in the semiconductor industry to have a method that allows for inspection of image sensor wafers that have front surfaces covered by a protective layer.